1. Field of the Invention
The present invention relates to the field of magnetic resonance (MR) imaging and, particularly, to a method and system for echo planar imaging.
2. Description of the Prior Art
The principles of magnetic resonance imaging (MRI) are that when an external magnetic field is applied, radiofrequency (RF) pulses of a specific frequency are employed to excite the protons in a tissue under examination, the protons absorbing certain energy to resonate; after the RF pulse emission is stopped, the excited protons gradually release the absorbed energy in the form of scanning signals, and a scanned image of the tested tissue can be obtained by collecting the scanning signals and employing an image reconstruction technique to process the scanning signals. It needs to be pointed out that, the basic unit for signal processing is the voxel, a voxel can include one or more protons, and when image reconstruction is carried out, the processed object is the scanning signal collected from each voxel.
In this case, the external magnetic field includes a main (basic) magnetic field and three orthogonal gradient magnetic fields, and in these three fields the direction which is the same as the main magnetic field is generally defined as Z axis direction, and the X axis and Y axis are orthogonal with the Z axis. Specifically, the gradient magnetic field along the direction of the Z axis is referred to as a slice selection (SS) gradient, and, at the same time, the direction of the Z axis is also referred to as the SS direction; the gradient magnetic field along the direction of the Y axis is referred to as the phase encoding (PE) gradient, at the same time, the direction of the Y axis is also referred to as the PE direction; and the gradient magnetic field along the direction of the X axis is referred to as a frequency encoding gradient, which is also referred to as the readout (RO) gradient in practical applications, and the direction of the X axis is also referred to as the RO direction.
With the development of MRI technology, echo planar imaging (EPI) sequences are currently the sequences with the fastest scanning speed, and FIG. 1 is a schematic diagram of an EPI sequence in the prior art. As shown in FIG. 1, a slice selection gradient 101 is applied in the SS direction, then the voxels at different positions in the SS direction have different resonance frequencies; at this moment an RF pulse 102 at an angle of 90° is used to selectively excite the voxels at a certain frequency, and the voxels excited at the certain frequency are in one slice; then a phase encoding gradient 103 is applied in the PE direction, so the voxels at different positions in the PE direction have phase differences; and, at the same time, a readout gradient 104 is applied in the RO direction, then the protons at different positions in the RO direction would attenuate at different frequencies. In this case, the greatest feature of an EPI sequence is that the readout gradients 104 are continuous and alternate between positive and negative; an echo collection, i.e. a collection of scanning signals, is carried out during the period of each readout gradient 104, and a phase encoding gradient 103 is applied before starting each echo collection.
However, in practical applications, since the effects of factors, such as eddy currents, the nonuniformity of the main magnetic field or the nonuniformity of the magnetic susceptibility thereof, etc. would cause the nonuniformity of the magnetic field, the resonance frequency of a voxel would change, and when a frequency encoding and a phase encoding are carried out, errors may occur in the phase of the voxel, which is especially serious during the phase encoding in an EPI sequence; therefore, although an EPI sequence can carry out fast imaging, the image distortions resulting from the nonuniformity of the magnetic field cannot be overcome. For these reasons, a series of improvements have been made on the basis of employing the currently available EPI sequence to carry out a scan, and there are mainly the following improvement methods. Firstly, after having scanned by employing an EPI sequence, re-scanning is carried out by employing a bi-gradient echo sequence, and after this re-scanning has been carried out by employing a bi-gradient echo sequence, the nonuniform field pattern of the magnetic field can be obtained, from which the number of the displaced pixels of each voxel in the scanned image can be deduced, and then the scanned image obtained by the EPI sequence is corrected; secondly, one scan is carried out by employing an EPI sequence on the basis of a phase encoding gradient with positive polarity, and then one scan is carried out by employing an EPI sequence on the basis of a phase encoding gradient with negative polarity, thus two scanned images can be obtained, and in these two images, the directions of the displacements occurring in the phase encoding direction due to the nonuniformity of the magnetic field are opposite, and the number of the displaced pixels of each voxel in the scanned images can be deduced by calculating the integration of the voxel phantom of each voxel in the phase encoding direction in the two scanned images. It needs to be pointed out that, FIG. 1 shows an EPI sequence on the basis of a phase encoding gradient with positive polarity, and, in the EPI sequence on the basis of a phase encoding gradient with negative polarity, the direction of the phase encoding gradient would be opposite to the direction of the phase encoding gradient shown in FIG. 1; thirdly, the effective bandwidth of the phase encoding gradient is increased, in which the effective bandwidth of the phase encoding gradient is in proportion to the reciprocal of the time of each echo collection, while the degree of the image distortion is in inverse proportion to the effective bandwidth in the phase encoding direction. For example, assuming the effective bandwidth of the phase encoding gradient is 10 Hz and the frequency difference (the difference between the actual resonance frequency and the uniform field resonance frequency) due to the nonuniformity of the magnetic field is 20 Hz, then the number of resulting displaced pixels is 20/10=2; and assuming the effective bandwidth of the phase encoding gradient to be increased to 20 Hz and the frequency difference due to the nonuniformity of the magnetic field to stay the same, then the number of resulting displaced pixels is now 20/20=1.
However, there are still defects in the abovementioned three improvement methods: in the first and the second improvement methods the scanning time is increased, and in the first improvement method, when the nonuniformity of the magnetic field is relatively large, there are deviations in shape between the nonuniform field pattern of the magnetic field and the EPI scanned image itself, thus the number of displaced pixels of each proton deduced from the nonuniform field pattern of the magnetic field is inaccurate; while in the second improvement method, the noises of the two scans are different, thus when the integration of each voxel in the phase encoding direction is carried out in the two scanned images, the effects caused by the noise signals are different, so the number of displaced pixels of the proton deduced from the two images is inaccurate; and the third scanning method can be realized in theory, but the maximum of the effective bandwidth in the phase encoding direction is limited by hardware devices such as the gradient system and so on, so the effective bandwidth in the phase encoding direction cannot be increased unlimitedly; therefore it can be seen that none of the three improvement methods can avoid image distortions effectively.